The Collaborative International Dictionary of English v.0.48:

Unilateral \U`ni*lat"er*al\, a. [Uni- + lateral: cf. F.
   unilat['e]ral.]
   1. Being on one side only; affecting but one side; one-sided.
      [1913 Webster]

   2. (Biol.) Pertaining to one side; one-sided; as, a
      unilateral raceme, in which the flowers grow only on one
      side of a common axis, or are all turned to one side.
      [1913 Webster]

   Unilateral contract (Law), a contract or engagement
      requiring future action only by one party.
      [1913 Webster]

WordNet (r) 3.0 (2006):

unilateral contract
    n 1: a one-sided agreement whereby you promise to do (or refrain
         from doing) something in return for a performance (not a
         promise)